1. Field of the Invention
The present invention relates to a method for producing a latent electrostatic image developing carrier that is favorably used for electrophotographic method, electrostatic recording method, and electrostatic printing method, and also relates to a method for producing the carrier, a developer using the carrier, and an image forming method using the developer.
2. Description of the Related Art
In a dry developing method used for electrophotography, a visible image is formed by making a toner frictioned with a charging member electrostatically adhered on a latent electrostatic image. In such a dry developing method, there are two types of developing methods. One is one-component developing method using a one-component developer composed of a toner, and another is two-component developing method composed of a glass bead, a magnetic carrier or a carrier covered with a resin etc. on the surface thereof, and a toner.
In a developer used in the two-component developing method, a fine toner is held on the surface of a relatively large core material by an electric force generated by friction between the toner and the core material. When the developer is closely situated on a latent electrostatic image, the toner overcomes a bonding force with the core material to thereby be developed on the latent electrostatic image. Then, the developer is repeatedly used while a toner consumed in developing being refilled. For this reason, the toner must be made to constantly have a desired polarity and must be frictionally charged with a sufficient charge amount on the core material during use of the core material. However, a toner fused on the surface of the core material, which is called “spent toner”, by inter-particle collision and mechanical agitation between particles and a developing device used or heat generation, and thus the charging property of the core material degrades with use time. Consequently, background smear and toner scattering occur on images. Therefore, there is a need to entirely replace the developer.
To prevent occurrence of such a spent toner, there have been a number techniques of prolonging operation life of a carrier proposed so far by covering the surface of a core material with a resin having a low-surface energy such as a fluorine resin and a silicone resin. For example, a carrier covered with a room temperature curable silicone resin and a positively charged nitrogen resin (see Japanese Patent Application Laid-Open (JP-A) No. 55-127569), a carrier covered with a cover material containing at least one modified silicone resin (see Japanese Patent Application Laid-Open (JP-A) No. 55-157751), a carrier having a coating layer containing a room temperature curable silicone resin and a styrene-acrylic resin (see Japanese Patent Application Laid-Open (JP-A) No. 56-140358), a carrier in which the surface of a core particle is covered with two or more layers composed of a silicone resin so as not to have adhesion property between the layers (Japanese Patent Application Laid-Open (JP-A) No. 57-96355), a carrier in which the surface of a core particle is covered with multiple layers of a silicone resin (see Japanese Patent Application Laid-Open (JP-A) No. 57-96356), a carrier whose surface is covered with a silicone resin containing a silicon carbide (see Japanese Patent Application Laid-Open (JP-A) No. 58-207054), a positively charged carrier covered with a material exhibiting a critical surface tension of 20 dyn/cm or less (Japanese Patent Application Laid-Open (JP-A) No. 61-110161), and a developer containing a carrier covered with a coating material containing fluorine alkyl acrylate and a toner containing a chrome-contained azo dye (see Japanese Patent Application Laid-Open (JP-A) No. 62-273576).
Recently, to obtain a high-quality image, a toner tends to have a smaller diameter. As a result, a spent toner easily occurs onto a carrier. Further, with a conventional spray coating method, it is difficult to uniformly damp a carrier surface with a coating material. Therefore, it is difficult to produce a carrier having appropriate adhesion property between a coating layer and a core material and having a uniform thickness and uniform film quality. Further, in a case of a full-color toner, to obtain a sufficient color tone, a resin having a low-softening point is used. Accordingly, a full-color toner causes a carrier spent amount more than that of a black toner, and the toner charge amount is reduced to cause toner scattering and background smear. As described above, a full-color electrophotographic system has a problem that when the toner charge amount is reduced, the image density particularly in highlight parts is easily changed and a high-image quality cannot be maintained.
Further, to improve the durability of a carrier, technique are proposed in which a coating layer is formed in which a fine particle and a conductivity imparting material are dispersed in the resin matrix of a low-surface energy material to thereby control the spent resistance, film strength, and electric properties (see Japanese Patent Application Laid-Open (JP-A) Nos. 9-319161, 9-269614 and 10-186731). However, these proposals have a problem that the toner charge amount is hardly uniformized due to flocculated fine particles etc. because a dispersion prepared by adding the fine particles into an organic solvent is spray-coated at a high temperature.
Furthermore, conventional spray-coating methods have a production problem that it is necessary to clear the regulation of volatile organic compounds (VOC) and waste water that could be produced in an organic solvent used in forming a coating layer on the surface of a core material, and thus a dry energy is required. To address these problems, a method of producing a carrier is proposed which uses a supercritical fluid in a dry powder process without using, for example, an organic solvent (see U.S. Pat. No. 5,514,512). However, the proposal cannot solve the above-mentioned problem with a spent toner, because in the proposal, a coating resin polymerized in a supercritical fluid is heated and fused in a supercritical fluid and the surface of a core material is covered with the fluid.
Further, in a method of producing a carrier using a supercritical fluid described in Japanese Patent Application Laid-Open (JP-A) No. 2006-106208, a silicone resin is used as a coating material, but the solubility of the silicone resin to the supercritical fluid is insufficient, and then a plasticized and finely dispersed silicone resin is sprayed together with a core material, thereby coating the surface of a core material. As a result, the obtainable coating layer is poor in thickness uniformity, and the method has a problem that it is impossible to form a coating layer having a thickness required to satisfy the durability.
Accordingly, there has not yet been provided a carrier that has a coating layer having a uniform and appropriate thickness on the surface of a core material and has a high adhesion property between the core material and the coating layer and sufficiently satisfiable related techniques thereof. Further, from the perspective of environmental burden and resource-saving, the current situation is that conventional methods of producing a carrier and conventional carriers have various problems to solve.